[Rails] AR validation question

What is the best way to implement a validation for a withdrawal operation? So there is an Operation and Client models.
Client has_many :operations. In case of withdrawal operation, I'd like to check if the requested sum is not greater than a Client account balance. How to link the validation in the Operation model to a specific client ?

class Client < AR
  has_many :operations
end

class Operation < AR
  belongs_to :client
  attr_accessible :sum
end

The routes are defined so that an operation could only created in the context of a Client.

Thanks

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